Meniscal xenografts

ABSTRACT

The invention provides an article of manufacture comprising a substantially non-immunogenic knee meniscal xenograft for implantation into humans. The invention further provides methods for preparing a knee meniscal xenograft by removing at least a portion of a meniscus from a non-human animal to provide a xenograft; washing the xenograft in saline and alcohol; and subjecting the xenograft to at least one treatment selected from the group consisting of exposure to ultraviolet radiation, immersion in alcohol, ozonation, and freeze/thaw cycling. In addition to or in lieu of the above treatments, the methods include a cellular disruption treatment and either digestion of the carbohydrate moieties of the xenograft with a glycosidase in a range of about 1 mU/ml to about 1000 U/ml or glycosidase digestion followed by treatment for sialylation. The invention also provides articles of manufacture produced by one or more of the above-identified methods of the invention. The invention further provides a meniscal xenograft for implantation into a human including a portion of a meniscus from a non-human animal, wherein the portion includes extracellular matrix and substantially only dead cells. The matrix and dead cells have substantially no surface α-galactosyl moieties and have sialic acid molecules linked to at least a portion of surface carbohydrate moieties. Each of the xenografts of the invention is substantially non-immunogenic and has substantially the same mechanical properties as the respective native meniscus.

RELATED APPLICATION

This application is a continuation-in-part of application U.S. Ser. No. 08/483,256, filed Jun. 7, 1995 now U.S. Pat. No. 5,865,849.

FIELD OF THE INVENTION

The present invention relates to the field of treatment of injured human knee joints, and in particular, to replacement and repair of a damaged human knee joint meniscus using a substantially immunologically compatible meniscus from a non-human animal.

BACKGROUND OF THE INVENTION

The human knee is a complex joint containing spatially interrelated bones, ligaments, and cartilaginous structures which interact to create a variety of motions. Specifically, the femoral condyles articulate with the surface plateaus of the tibia, through the cartilaginous medial and lateral menisci, and all of these structures are held in place by various ligaments. By virtue of their cartilaginous nature, the medial and lateral menisci are structures comprised of cells called fibrochondrocytes and an extracellular matrix of collagen and elastic fibers as well as a variety of proteoglycans. Undamaged menisci provide shock absorption for the knee by ensuring proper force distribution, stabilization, and lubrication for the interacting bone surfaces within the knee joint, which are routinely exposed to repeated compression loading during normal activity. Much of the shock absorbing function of the medial and lateral menisci is derived from the elastic properties inherent to cartilage. When menisci are damaged through injury, disease, or inflammation, arthritic changes occur in the knee joint, with consequent loss of function.

Since joint cartilage in adults does not naturally regenerate to a significant degree once it is destroyed, damaged adult menisci have historically been treated by a variety of surgical interventions including removal and replacement with prosthetic devices. An artificial knee joint having a rigid plastic femoral member and a metal tibial member is disclosed in U.S. Pat. No. 4,034,418. A number of meniscus prostheses have been devised which employ resilient materials such as silicone rubber or natural rubber, as in U.S. Pat. No. 4,344,193 and U.S. Pat. No 4,502,161. Additional deformable, flexible resilient materials for a meniscus prosthesis such as collagen, tendon, or fibrocartilage are disclosed in U.S. Pat No. 5,092,894 and U.S. Pat. No. 5,171,322. A cartilage replacement apparatus constructed of polyethylene plastic filled with small ball bearings or gelatinous fluid is described in U.S. Pat No. 5,358,525. However, the known artificial prostheses have been unsatisfactory for treatment of damaged menisci, since they are deficient in the elastic, and therefore in the shock-absorbing, properties characteristic of natural menisci. Moreover, the known artificial devices have not proven able to withstand the forces inherent to routine knee joint function.

The present inventor provided improved prosthetic menisci in several of his earlier patents (U.S. Pat No. 4,880,429; U.S. Pat. No. 5,007,934; U.S. Pat. No. 5,116,374; and U.S. Pat. No. 5,158,574). These patents generally disclose prosthetic menisci formulated from dry, porous matrices of processed natural fibers such as reconstituted cross-inked collagen, which optionally include glycosaminoglycan molecules. Generally, the source of collagen for these prosthetic menisci has been animal Achilles tendons or skin. The reconstitution process removes non-collagenous materials such as glycoproteins, proteoglycans, lipids, native glycosaminoglycans, and the like, which may confer additional elastic properties to the original tissue.

Much of the structure and many of the properties of original tissues may be retained in transplants through use of heterograft or xenograft materials, that is, tissue from a different species than the graft recipient. For example, tendons or ligaments from cows or other animals are covered with a synthetic mesh and transplanted into a heterologous host in U.S. Pat. No. 4,400,833. Flat tissues such as pig pericardia are also disclosed as being suitable for heterologous transplantation in U.S. Pat. No. 4,400,833. Bovine peritoneum fabricated into a biomaterial suitable for prosthetic heart valves, vascular grafts, burn and other wound dressings is disclosed in U.S. Pat. No. 4,755,593. Bovine, ovine, or porcine blood vessel xenografts are disclosed in WO 84/03036. However, none of these disclosures describe the use of a xenograft for meniscus replacement.

Once implanted in an individual, a xenograft provokes immunogenic reactions such as chronic and hyperacute rejection of the xenograft. The term "chronic rejection", as used herein refers to an immunological reaction in an individual against a xenograft being implanted into the individual. Typically, chronic rejection is mediated by the interaction of IgG natural antibodies in the serum of the individual receiving the xenograft and carbohydrate moieties expressed on cells, and/or cellular and/or extracellular matrices of the xenograft. For example, transplantation of cartilage xenografts from nonprimate mammals (e.g., porcine or bovine origin) into humans is primarily prevented by the interaction between the IgG natural anti-Gal antibody present in the serum of humans with the carbohydrate structure Galα1-3Galβ1-4GlcNAc-R (α-galactosyl or α-gal epitope) expressed in the xenograft K. R. Stone et al., Porcine and bovine cartilage transplants in cynomolgus monkey: I. A model for chronic xenograft rejection, 63 Transplantation 640-645 (1997); U. Galili et al., Porcine and bovine cartilage transplants in cynomolgus monkey: II. Changes in anti-Gal response during chronic rejection, 63 Transplantation 646-651 (1997). In chronic rejection, the immune system typically responds within one to two weeks of implantation of the xenograft.

In contrast with "chronic rejection", "hyper acute rejection" as used herein, refers to the immunological reaction in an individual against a xenograft being implanted into the individual, where the rejection is typically mediated by the interaction of IgM natural antibodies in the serum of the individual receiving the xenograft and carbohydrate moieties expressed on cells. This interaction activates the complement system causing lysis of the vascular bed and stoppage of blood flow in the receiving individual within minutes to two to three hours.

The term "extracellular matrix or matrices", as used herein, refer to collagen and elastic fibers, as well a variety of proteoglycans, which are secreted by fibrochondrocytes during cartilage growth and which undergo slow turn-over.

Xenograft materials may be chemically treated to reduce immunogenicity prior to implantation into a recipient. For example, glutaraldehyde is used to cross-link or "tan", xenograft tissue in order to reduce its antigenicity, as described in detail in U.S. Pat No. 4,755,593. Other agents such as aliphatic and aromatic diamine compounds may provide additional crosslinking through the side chain carboxyl groups of aspartic and glutamic acid residues of the collagen polypeptide. Glutaraldehyde and diamine tanning also increases the stability of the xenograft tissue.

Xenograft tissues may also be subjected to various physical treatments in preparation for implantation. For example, U.S. Pat. No. 4,755,593 discloses subjecting xenograft tissue to mechanical strain by stretching to produce a thinner and stiffer biomaterial for grafting. Tissue for allograft transplantation is commonly cryopreserved to optimize cell viability during storage, as disclosed, for example, in U.S. Pat. No. 5,071,741; U.S. Pat. No. 5,131,850; U.S. Pat. No. 5,160,313; and U.S. Pat. No. 5,171,660. U.S. Pat. No. 5,071,741 discloses that freezing tissues causes mechanical injuries to cells therein because of extracellular or intracellular ice crystal formation and osmotic dehydration.

SUMMARY OF THE INVENTION

The present invention provides a substantially non-immunogenic meniscal cartilage xenograft for implantation into a human in need of knee meniscus repair or replacement. The invention further provides methods for processing xenogeneic meniscal cartilage with reduced immunogenicity but with substantially native elasticity and load-bearing capabilities for xenografting into humans.

As described herein, the term "xenograft" is synonymous with the term "heterograft" and refers to a graft transferred from an animal of one species to one of another species. Stedman's Medical Dictionary, Williams & Wilkins, Baltimore, Md. (1995).

As described herein, the term "xenogeneic", as in xenogeneic graft, cartilage, etc., refers to a graft, cartilage, etc., transferred from an animal of one species to one of another species. Id.

The methods of the invention, include, alone or in combination, treatment with radiation, one or more cycles of freezing and thawing, treatment with a chemical cross-linking agent, treatment with alcohol, or ozonation. In addition to or in lieu of these methods, the methods of the invention include a cellular disruption treatment and digestion of the carbohydrate moieties of the xenograft with a glycosidase in a concentration range of about 1 mU/ml to about 1000 U/ml or glycosidase digestion followed by treatment of the carbohydrate moieties of the xenograft with sialic acid. After one or more of the above-described processing steps, the methods of the invention provide a xenograft having substantially the same mechanical properties as a native knee meniscus.

As described herein, the term "cellular disruption" as in, for example, cellular disruption treatment, refers to a treatment for killing cells.

As described herein, the term "capping molecules", refers to molecules which link with carbohydrate chains such that the xenograft is no longer recognized as foreign by the subject's immune system.

In one embodiment, the invention provides an article of manufacture comprising a substantially non-immunogenic knee meniscal xenograft for implantation into a human.

In another embodiment, the invention provides a method of preparing a meniscal xenograft for implantation into a human, which includes removing at least a portion of a medial or lateral meniscus from a knee joint of a non-human animal to provide a xenograft; washing the xenograft in water and alcohol; and subjecting the xenograft to at least one treatment selected from the group consisting of exposure to ultraviolet radiation, immersion in alcohol, ozonation, and freeze/thaw cycling, whereby the xenograft has substantially the same mechanical properties as the native meniscus.

As described herein, the term "portion", as in, for example, a portion of medial or lateral meniscus or a portion of surface carbohydrate moieties refers to all or less than all of the respective medial or lateral meniscus or surface carbohydrate moieties.

In still another embodiment, the invention provides a method of preparing a meniscal xenograft for implantation into a human, which includes removing at least a portion of medial or lateral meniscus from a knee joint of a non-human animal to provide a xenograft; washing the xenograft in water and alcohol; subjecting the xenograft to a cellular disruption treatment; and digesting the xenograft with a glycosidase in a concentration range of about 1 mU/ml to about 1000 U/ml to remove substantially first surface carbohydrate moieties from the xenograft, whereby the xenograft has substantially the same mechanical properties as the native meniscus.

In a further embodiment, the invention provides a method of preparing a meniscal xenograft for implantation into a human, which includes removing at least a portion of a medial or lateral meniscus from a knee joint of a non-human animal to provide a xenograft; washing the xenograft in water and alcohol; subjecting the xenograft to a cellular disruption treatment; digesting the xenograft with a glycosidase to remove substantially first surface carbohydrate moieties from the xenograft; and treating second surface carbohydrate moieties on the xenograft with sialic acid to cap at least a portion of the second surface carbohydrate moieties, whereby the xenograft is substantially non-immunogenic and has substantially the same mechanical properties as the native meniscus.

As described herein, the terms "to cap" or "capping", refer to linking a carboydrate unit to the end of a carbohydrate chain, as in, for example, covalently linking salic acid to surface carbohydrate moieties on the xenograft.

In still further embodiments, the invention provides articles of manufacture including substantially non-immunogenic knee meniscal xenografts for implantation into humans produced by one or more of the above-identified methods of the invention.

In yet another embodiment, the invention provides a meniscal xenograft for implantation into a human which includes a portion of a medial or lateral meniscus from a knee joint of a non-human animal, wherein the portion includes an extracellular matrix and substantially only dead cells, the extracellular matrix and the dead cells having substantially no surface α-galactosyl moieties and having sialic acid molecules linked to at least a portion of surface carbohydrate moieties. The portion of the medial or lateral meniscus is substantially non-immunogenic and has substantially the same mechanical properties as the native meniscus.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features of the invention may be more fully understood from the following description when read together with the accompanying drawings.

FIG. 1 shows a simplified diagrammatic representation of a human knee joint, with medial and lateral menisci in their native positions.

FIG. 2 is a diagrammatic representation of a cut-away view of a human knee joint, showing the medial and lateral menisci as they are positioned in vivo over the medial and lateral condyles of the tibia.

FIG. 3 is a diagrammatic representation of resection of a torn lateral meniscus of a human knee, and preparation of the knee for receipt of a meniscal implant.

FIG. 4 is a diagrammatic representation the preferred drill guide placement for posterior lateral meniscal horn insertion into a human knee.

FIG. 5 is a diagrammatic representation of a cannulated drill overdrilling guide wire at the posterior lateral meniscal horn insertion into a human knee.

FIG. 6 is a diagrammatic representation of the appearance of a human knee with posterior and anterior drill holes for meniscal horn insertion.

FIG. 7 is a diagrammatic representation of the preferred suture passer placement for pulling a meniscal implant into a human knee joint.

FIG. 8 is a diagrammatic representation of the appearance of a human knee containing a meniscal implant during the insertion stage.

FIG. 9 is a diagrammatic representation of the appearance of a human knee containing a meniscal implant with zone-specific meniscal repair sutures in place for final tying of the meniscal repair sutures.

FIG. 10 is a graphical representation of the specificity of monoclonal anti-Gal antibodies for α-galactosyl epitopes on bovine serum albumin (BSA), bovine thyroglobulin, mouse laminin, Galβ1-4 G1cNAc-BSA (N-acetyllactosamine-BSA), Galα1-4Galβ1-4G1cNAc-BSA P1 antigen linked to BSA), and human thyroglobulin or human laminin.

FIG. 11 is a graphical representation of α-galactosyl epitope elimination from α-galactosidase treated cartilage.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed against the chronic rejection of xenografts for implantation into humans. Accordingly, the meniscal xenograft produced in accordance with the method of the invention is substantially non-immunogenic, while generally maintaining the mechanical properties of a native meniscus.

While the meniscus may undergo some shrinkage during processing, a medial meniscus xenograft prepared in accordance with the invention will have the general appearance of a native medial meniscus, and a lateral meniscus xenograft of the invention will have the general appearance of a native lateral meniscus. The meniscal xenograft may also be cut into segments, each of which may be implanted into the knee of a recipient as set forth below.

The invention provides, in one embodiment, a method for preparing or processing a xenogeneic medial or lateral meniscus for engraftment into humans. The knee menisci may be harvested from any non-human animal to prepare the xenografts of the invention. Menisci from transgenic non-human animals or from genetically altered non-human animals may also be used as xenografts in accordance with the present invention. Preferably, bovine, ovine, or porcine knee joints serve as sources of the medial and lateral menisci used to prepare the xenografts. More preferably, immature pig, calf or lamb knee joints are the sources of the menisci, since the cartilage of younger animals may be inherently more elastic and engraftable than that of older animals. Most preferably, the age of the source animal is between six and eighteen months at time of slaughter.

In the first step of the method of the invention, an intact medial or lateral meniscus is removed from the knee of a non-human animal. The joint which serves as the source of the menisci should be collected from freshly killed animals and preferably immediately placed in a suitable sterile isotonic or other tissue preserving solution. Harvesting of the joints should occur as soon as possible after slaughter of the animal and preferably should be performed in the cold, i.e., in the approximate range of about 5° C. to about 20° C., to minimize enzymatic degradation of the meniscal tissue.

The menisci are harvested from the joints in the cold, under strict sterile technique. The joint is opened by first transecting the patellar tendon. The horns of the menisci are dissected free of adhering tissue. A small amount of bone representing a substantially cylindrical plug of approximately five millimeters in diameter by five millimeters in depth may be left attached to the horns. The meniscal synovial junction is carefully identified and freed from the meniscus tissue itself, thereby forming the xenograft.

The xenograft is then washed in about ten volumes of sterile cold water to remove residual blood proteins and water soluble materials. The xenograft is then immersed in alcohol at room temperature for about five minutes, to sterilize the tissue and to remove non-collagenous materials. A xenograft appears as a shiny "C"-shaped fibrous tissue, having generally a crescent-shaped principal surface on the top side (the "superior surface") and a generally crescent-shaped principal surface on the bottom side (the "inferior surface"), where the outer portions of the superior and inferior surfaces are joined by way of an outer lateral surface and the inner portions of the superior and inferior surfaces are joined by way of an inner lateral surface.

After alcohol immersion, the xenograft may be subjected to at least one of the following treatments: radiation treatment, treatment with alcohol, ozonation, one or more cycles of freezing and thawing, and/or treatment with a chemical cross-linking agent. When more than one of these treatments is applied to the xenograft, the treatments may occur in any order.

In one embodiment of the method of the invention, the xenograft may be treated by exposure to ultraviolet radiation for about fifteen minutes or gamma radiation in an amount of about 0.5 to 3 MegaRad.

In another embodiment, the xenograft may be treated by again being placed in an alcohol solution. Any alcohol solution may be used to perform this treatment. Preferably, the xenograft is placed in a 70% solution of isopropanol at room temperature.

In still another embodiment, the xenograft may be subjected to ozonation.

In a further embodiment of the method of the invention, the xenograft may be treated by freeze/thaw cycling. For example, the xenograft may be frozen using any method of freezing, so long as the xenograft is completely frozen, i.e., no interior warm spots remain which contain unfrozen tissue. Preferably, the xenograft is dipped into liquid nitrogen for about five minutes to perform this step of the method. More preferably, the xenograft is frozen slowly by placing it in a freezer. In the next step of the freeze/thaw cycling treatment, the xenograft is thawed by immersion in an isotonic saline bath at room temperature (about 25° C.) for about ten minutes. No external heat or radiation source is used, in order to minimize fiber degradation.

In yet a further embodiment, the xenograft may optionally be exposed to a chemical agent to tan or crosslink the proteins within the extracellular matrix, to further diminish or reduce the immunogenic determinants present in the xenograft. Any tanning or crosslinking agent may be used for this treatment, and more than one crosslinking step may be performed or more than one crosslinking agent may be used in order to ensure complete crosslinking and thus optimally reduce the immunogenicity of the xenograft. For example, aldehydes such as glutaraldehyde, formaldehyde, adipic dialdehyde, and the like, may be used to crosslink the collagen within the extracellular matrix of the xenograft in accordance with the method of the invention. Other suitable crosslinking agents include aliphatic and aromatic diamines, carbodiimides, diisocyanates, and the like. When glutaraldehyde is used as the crosslinking agent, for example, the xenograft may be placed in a buffered solution containing about 0.05 to about 5.0% glutaraldehyde and having a pH of about 7.4. Any suitable buffer may be used, such as phosphate buffered saline or trishydroxymethylaminomethane, and the like, so long as it is possible to maintain control over the pH of the solution for the duration of the crosslinking reaction, which may be from one to fourteen days, and preferably from three to five days. Alternatively, the xenograft can be exposed to a crosslinking agent in a vapor form, including, but not limited to, a vaporized aldehyde crosslinking agent, such as, for example, vaporized formaldehyde. The vaporized crosslinking agent can have a concentration and a pH and the xenograft can be exposed to the vaporized crosslinking agent for a period of time suitable to permit the crosslinking reaction to occur. For example, the xenograft can be exposed to vaporized crosslinking agent having a concentration of about 0.05 to about 5.0% and a pH of about 7.4, for a period of time which can be from one to fourteen days, and preferably from three to five days. Exposure to vaporized crosslinking agent can result in reduced residual chemicals in the xenograft from the crosslinking agent exposure. The crosslinking reaction should continue until the immunogenic determinants are substantially removed from the xenogeneic tissue, but the reaction should be terminated prior to significant alterations of the mechanical properties of the xenograft. When diamines are also used as crosslinking agents, the glutaraldehyde crosslinking should occur after the diamine crosslinking, so that any unreacted diamines are capped. After the crosslinking reactions have proceeded to completion as described above, the xenograft should be rinsed to remove residual chemicals, and 0.01-0.05 M glycine may be added to cap any unreacted aldehyde groups which remain.

In addition to or in lieu of the above treatments, the xenograft can be subjected to a cellular disruption treatment to kill the xenograft's fibrochondrocytes, which precedes or follows digestion of the xenograft with glycosidases to remove surface carbohydrate moieties from the xenograft. The glycosidase concentration is in a range about 1 mU/ml to about 1000 U/ml, and preferably, in the range of about 10 U/ml to about 500 U/ml, and most preferably, in the range of about 100 U/ml to 200 U/ml. The glycosidase digestion in turn can be followed by linkage with capping molecules such as sialic acid to cap surface N-acetyllactosamine ends of carbohydrate chains of the xenograft.

In an embodiment of this method of the invention, the xenograft is subjected to a cellular disruption treatment to kill the fibrochondrocytes of the meniscus cartilage prior to in vitro digestion of the xenograft with glycosidases. Typically after surface carbohydrate moieties have been removed from nucleated cells and the extracellular matrix, the nucleated, i.e., living cells reexpress the surface carbohydrate moieties. Reexpression of antigenic moieties of a xenograft can provoke continued immunogenic rejection of the xenograft. In contrast, non-nucleated, i.e., dead cells, are unable to reexpress surface carbohydrate moieties. Removal of antigenic surface carbohydrate moieties from the non-nucleated cells and the extracellular matrix of a xenograft substantially permanently eliminates antigenic surface carbohydrate moieties as a source of immunogenic rejection of the xenograft.

Accordingly, in the above-identified embodiment, the xenograft of the present invention is subjected to freeze/thaw cycling as discussed above to disrupt, i.e., to kill the fibrochondrocytes of the meniscus cartilage. Alternatively, the xenograft of the present invention is treated with gamma radiation having an amount of 0.2 MegaRad up to about 3 MegaRad. Such radiation kills the fibrochondrocytes and sterilizes the xenograft. Once killed, the fibrochondrocytes are no longer able to reexpress antigenic surface carbohydrate moieties such α-gal epitopes which are factors in the immunogenic rejection of the transplanted xenografts.

Either before or after the fibrochondrocytes are killed, the xenograft is subjected to in vitro digestion of the xenograft with glycosidases, and specifically galactosidases, such as α-galactosidase, to enzyimatically eliminate antigenic surface carbohydrate moieties. In particular, α-gal epitopes are eliminated by enzymatic treatment with α-galactosidases, as shown in the following reaction: ##STR1## The N-acetyllactosamine residues are epitopes that are normally expressed on human and mammalian cells and thus are not immunogenic. The in vitro digestion of the xenograft with glycosidases is accomplished by various methods. For example, the xenograft can be soaked or incubated in a buffer solution containing glycosidase. In addition, the xenograft can be pierced to increase permeability, as further described below. Alternatively, a buffer solution containing the glycosidase can be forced under pressure into the xenograft via a pulsatile lavage process.

Elimination of the α-gal epitopes from the xenograft diminishes the immune response against the xenograft. The α-gal epitope is expressed in nonprimate mammals and in New World monkeys (monkeys of South America) as 1×10⁶ -35×10⁶ epitopes per cell, as well as on macromolecules such as proteoglycans of the extracellular matrix. U. Galili et al., Man, apes, and Old World monkeys differ from other mammals in the expression of α-galactosyl epitopes on nucleated cells, 263 J. Biol. Chem. 17755 (1988). This epitope is absent in Old World primates (monkeys of Asia and Africa and apes) and humans, however. Id Anti-Gal is produced in humans and primates as a result of an immune response to α-gal epitope carbohydrate structures on gastrointestinal bacteria. U. Galili et al., Interaction between human natural anti-α-galactosyl immunoglobulin G and bacteria of the humanflora, 56 Infect. Immun. 1730 (1988); R. M. Hamadeh et al., Human natural anti-Gal IgG regulates alternative complement pathway activation on bacterial surfaces, 89 J. Clin. Invest. 1223 (1992). Since nonprimate mammals produce α-gal epitopes, xenotransplantation of xenografts from these mammals into primates results in rejection because of primate anti-Gal binding to these epitopes on the xenograft. The binding results in the destruction of the xenograft by complement fixation and by antibody dependent cell cytotoxicity. U. Galili et al., Interaction of the natural anti-Gal antibody with α-galactosyl epitopes: A major obstacle for xenotransplantation in humans, 14 Immunology Today 480 (1993); M. Sandrin et al., Anti-pig IgM antibodies in human serum react predominantly with Galα1-3Gal epitopes, 90 Proc. Natl. Acad. Sci. USA 11391 (1993); H. Good et al., Identification of carbohydrate structures which bind human anti-porcine antibodies: implications for discordant grafting in man 24 Transplant. Proc. 559 (1992); B. H. Collins et al., Cardiac xenografts between primate species provide evidence for the importance of the α-galactosyl determinant in hyperacute rejection, 154 J. Immunol. 5500 (1995). Furthermore, xenotransplantation results in major activation of the immune system to produce increased amounts of high affinity anti-Gal.

The cartilage xenografts of the present invention are particularly well suited to in vitro enzymatic elimination of the α-gal epitopes, however. In contrast to organs and other tissues, the cartilage extracellular matrix undergoes extremely slow turnover. Moreover, once the fibrochondrocytes are killed, these non-nucleated cells are prevented from reexpressing the α-gal epitopes, as discussed above. Accordingly, the substantial elimination of α-gal epitopes from cells and the extracellular matrix, and the prevention of reexpression of cellular α-gal epitopes can diminish the immune response against the xenograft associated with anti-Gal antibody binding with α-gal epitopes.

Following treatment with glycosidase, the remaining carbohydrate chains (e.g., glycosaminoglycans) of the xenograft are optionally treated with capping molecules to cap at least a portion of the remaining carbohydrate chains. This capping treatment involves capping molecules having a concentration range of about 0.1 mM to about 100 mM, and preferably, a concentration of about 0.1 mM to about 10 mM, and most preferably, a concentration of about 1 mM to about 4 mM. Treatment with capping molecules is applicable to both glycosidase-treated and non-glycosidase-treated xenografts. For example, xenografts from knock out animals which may lack α-gal epitopes may be treated with capping molecules to cap carbohydrate moieties on the xenograft, thereby reducing the xenograft's immunogenicity. Examples of capping molecules used in the present invention include fucosyl, n-acetyl glucosamine and sialic acid.

In addition, selected capping molecules, such as sialic acid, are negatively charged.

The replacement of α-gal epitopes with negatively charged molecules can further diminish immunogenic rejection of the xenograft. It is theorized that the decreased immnunogenicity of the xenograft results because the negative charges conferred by the capping molecules repel negatively charged antibody molecules and/or cells of the immune system, thereby masking immunogenic regions of the xenograft.

In general, electrostatic repulsion termed "zeta potential," prevents the interaction between molecules, other than ligands and their corresponding receptors, in the body, and serves as a barrier against nonspecific interactions. For example, sialic acid on carbohydrate chains of envelope glycoproteins helps infectious viruses to evade effective recognition by antibodies and by antigen presenting cells. T. W. Rademacher et al., Glycobiology, Ann. Rev. Biochem., 57:785 (1988). Bacteria such as Neisseria gonorrhea can prevent their immune destruction by coating themselves with sialic acid using a bacterial sialyltransferase R. F. Rest et al., Neisseria sialyltransferases and their role in pathogenesis, Microbial Pathogenesis, 19:379 (1995). Similarly, the protozoan Trypanosoma cruzi can infect humans and cause Chagas' disease because of effective sialylation of its cell surface glycoproteins with sialic acid by use of the enzyme transialidase which transfers sialic acid from host glycoproteins to carbohydrate chains on the parasite's membrane. O. Previato et al., Incorporation of sialic acid into Trypanosoma cruzi macromolecules, A proposalfor new metabolic route, Mol. Biochem. Parasitol., 16:85 (1985); B. Zingales et al., Direct sialic acid transfer from a protein donor to glycolipids of trypomastigote forms of Trypanosoma cruzi, Mol. Biochem. Parasitol., 26:1335 (1987). Decreasing immunogenicity by sialic acid is a method also used by mammalian cells. Normal antigen presenting cells prevent nonspecific adhesion with T lymphocytes by the expression of a highly sialylated protein named sialophorin (also termed CD43). E. Famole-Belasio et al., Antibodies against sialophorin (CD43) enhance the capacity of dendritic cells to cluster and activate T lymphocytes., J. Immunol., 159:2203 (1997). Many malignant cell types that acquire metastatic properties, increase the expression of sialic acid on their cell surface glycoproteins and thus mask their tumor antigens and decrease the possibility of their detection and destruction by the immune system. G. Yogeswarren et al., Metastatic potential is positively correlated with cell surface sialylation of cultural murine cells, Science, 212:1514 (1981); J. W. Dennis, Changes in glycosylation associated with malignant transformation and tumor progression. In: Cell surface carbohydrates and cell development, M. Fukuda, Ed. CRC Press, pp. 161-213 (1992).

The same strategy for prevention of immune recognition can be implemented by treatment of α-galactosidase treated xenografs with negatively charged molecules. Many of the antigens within a xenograft cartilage are effectively masked from the host immune system by the high density of carbohydrate chains. Most of these chains are negatively charged and are the same in the various mammalian species. D. Heinegard et al., Structure and biology of cartilage and bone matrix noncolagenous macromolecules, FASEB J., 3:2042 (1989); U. Lindahl et al., Glycosaminoglycans and their binding to biological macromolecules, Ann. Rev., Biochem, 47:385 (1978). Other antigens, however, are not masked by densely concentrated carbohydrate chains and thus may elicit an immune response against the xenograft. For example, some peptide regions, primarily in the proteoglycans of the cartilage extracellular matrix, are not masked by densely concentrated carbohydrate chains of keratin sulfate and chondroitin sulfate, and may elicit an immune response against the xenograft. The addition of negatively charged molecules to the ends of the carbohydrate chains on the cells and/or on the extracellular matrix molecules of the α-galactosidase treated xenografts can mask the non-α-Gal antigens of the xenograft and diminish immunogenic rejection of the xenograft.

Sialic acid is a non-limiting example of a negatively charged capping molecule used to cap the carbohydrate chains of the xenograft of the present invention. Sialic acid can be linked in vitro to carbohydrate chains of the xenograft by sialyltransferase (ST), preferably in a concentration of about 1 mU/ml to about 1000 U/ml, and more preferably in a concentration of about 10 U/ml to about 200 U/ml, in the following exemplary reaction: ##STR2## Sialic acid can also be linked in vitro to carbohydrate chains of the xenograft by recombinant trans-sialidase (TS), preferably in a concentration of about 1 mU/ml to about 1000 U/ml, and more preferably in a concentration of about 10 U/ml to about 200 U/ml, in the following exemplary reaction: ##STR3##

Prior to treatment, the outer lateral surface of the xenograft may optionally be pierced to increase permeability to agents used to render the xenograft substantially non-immunogenic. A sterile surgical needle such as an 18 gauge needle may be used to perform this piercing step, or, alternatively a comb-like apparatus containing a plurality of needles may be used. The piercing may be performed with various patterns, and with various pierce-to-pierce spacings, in order to establish a desired access to the interior of the xenograft. Piercing may also be performed with a laser. In one form of the invention, one or more straight lines of punctures about three millimeters apart are established circumferentially in the outer lateral surface of the xenograft.

Prior to implantation, meniscal xenograft of the invention may be treated with limited digestion by proteolytic enzymes such as ficin or trypsin to increase tissue flexibility, or coated with anticalcification agents, antithrombotic coatings, antibiotics, growth factors, or other drugs which may enhance the incorporation of the xenograft into the recipient knee joint. The meniscal xenograft of the invention may be further sterilized using known methods, for example, with additional glutaraldehyde or formaldehyde treatment, ethylene oxide sterilization, propylene oxide sterilization, or the like. The xenograft may be stored frozen until required for use.

The meniscal xenograft of the invention, or a segment thereof, may be implanted into damaged human knee joints by those of skill in the art using known arthroscopic surgical techniques. Specific instruments for performing arthroscopic techniques are known to those of skill in the art, which ensure accurate and reproducible placement of meniscal implants. For meniscal cartilage replacement to succeed, the following goals are preferably accomplished:

1. The torn fragmented pieces of native meniscal cartilage must be removed

2. The attachment sites for the meniscal horns must be anatomically placed.

3. The periphery of the meniscal implant must be attached securely enough to permit axial and rotational loads.

4. The surrounding capsule and ligaments of the knee joint must be neither excessively violated nor constrained by the fixation technique. The method of meniscal implantation described in detail below is derived from K. R. Stone, et al., Arthroscopy: The Journal of Arthroscopic and Related Surgery 9, 234-237 (1993); other methods of meniscal implantation may also be employed to use the xenogeneic meniscal xenografts of the present invention.

Initially, complete diagnostic arthroscopy of the knee joint is accomplished using known methods. If anterior cruciate ligament surgery is to be performed simultaneously, the femoral and tibial tunnels for the cruciate reconstruction should be drilled first. The torn portion of the meniscal cartilage is evaluated. If meniscal repair cannot be accomplished due to severity of the tear or poor quality of the tissue, then preparation of the meniscal rim is undertaken by removing the torn portions of the cartilaginous tissue (FIG. 3). When the entire human meniscus is to be replaced by a xenogeneic meniscus xenograft of the invention, nearly all of the human meniscus is removed. Additionally, for replacement of the entire human meniscus with a xenogeneic meniscus xenograft of the invention, resection of the human meniscal horns and preparation of bony tunnels to accept bone plugs may be required. When only a portion of the human meniscus is to be replaced with a segment of the xenogeneic meniscus xenograft of the invention, only the damaged portions are removed, preserving the peripheral rim and horns for attachment of the xenogeneic meniscus xenograft segment. If absolutely no human meniscal rim is present, then partial replacement of the meniscus should not be performed. If the joint is excessively tight, a joint distractor may be applied or the medial collateral ligament may be partially released.

For medial or lateral meniscal replacement, the arthroscope is placed in the mid-lateral or anterior lateral portal and the tibial guide is placed through the anterior medial portal. The tip of the guide is brought first to the posterior horn of the meniscus. It should be noted that the posteromedial horn inserts on the posterior slope of the tibial eminence. A drill pin is then brought from the anterior medial side of the tibial tuberosity to the posterior horn insertion (FIG. 4). The pin placement can be confirmed by passing the arthroscope through the intercondylar notch and viewing the exit site of the pin. Extreme care must be undertaken to avoid penetration through the posterior capsule of the knee, endangering the neurovascular bundle. When the pin position is confirmed, the pin is then overdrilled with a 4.5-mm cannulated drill bit with the option of a drill stop to prevent posterior capsular penetration (FIG. 5). The bit is left in place and used as a tunnel for passage of a suture passer with a suture such as a #2 Ethibond™ suture available from Johnson & Johnson. The suture is passed up the bore of the drill bit, the drill bit removed, and the suture left in place.

The anterior medial meniscus insertion point in humans varies considerably, most often being found anterior to the medial tibial eminence. The anterior horn of the lateral meniscus inserts just posterior to the anterior cruciate ligament. An anterior drill hole is made by first identifying the insertion point of the anterior horn of the lateral meniscus, by placing the tip of the drill guide so that a relatively vertical hole will be made (FIG. 6). The drill pin is placed, then the cannulated drill bit is used to overdrill the drill pin placement to form the anterior drill hole. A suture passer is placed in the anterior drill hole. Alternatively, the anterior horn of the medial meniscus is affixed with a suture anchor directly to bone as opposed to a drill hole.

Before the suture is grasped from the anterior and posterior drill holes, the anterior portal is widened to approximately 2 cm. The suture grasper is then passed through the widened portal, and both the anterior and the posterior sutures brought out simultaneously. This technique prevents the sutures from becoming entangled in two different planes of the fat pad and capsular tissue.

The implant is now brought onto the field. Two horizontal mattress sutures, for example, #2-0 Ethibond™ sutures or the like, are placed through each horn of the xenogeneic meniscus xenograft with the free ends exiting the inferior surface (FIG. 7). The two posterior sutures are then drawn through the knee and out the posterior tibial tunnel (FIG. 8). If viewing from a mid-lateral portal, the anterolateral portal can be used for probe insertion to push the implant medially into place through a 1-inch incision. No insertion cannula is required. The anterior sutures are then similarly passed. The horn sutures are then tied over the anterior tibial bony bridge.

Next, zone specific meniscal repair cannulae are brought into place. For medial insertions, a posterior medial vertical incision is made one third of the distance from the back of the knee for protection of the saphenous nerve and for retrieval of the inside-out meniscal repair needles. A second vertical incision is usually required further anteriorly, next to the anterior medial arthroscopy portal, to capture the anterior exiting needles. Through these two incisions, the suture needles can be captured and the knots placed directly over the capsule (FIG. 9).

When using the meniscal repair needles, the posterior cannulae should be used first, with the sutures placed vertically and evenly spaced. The repair should proceed from posterior to anterior so that a buckle is not produced within the xenograft. Each knot is tied as it is placed to avoid the chance of suture tangling. The knots are spaced approximately 4 mm apart. The knee is cycled through several complete ranges of motion of ensure that the xenograft moves smoothly without impingement.

When performing a lateral meniscal replacement, the medial portal is suitable for xenograft insertion. This may require excision of the ligamentous mucosa and removal of a portion of the fat pad. The drill guide for the posterior horn of the lateral meniscus is inserted through the anteromedial portal. The posterior slope of the lateral tibial spine must be identified for accurate meniscal horn insertion. The anterior horn inserts on the anterior slope of the lateral tibial spine in approximation to the lateral aspect of the anterior cruciate ligament. The advantage of drilling these holes from the medial side is that the tunnel divergence will be greater, providing a larger bony bridge between the horn insertions. The remainder of the insertion technique remains the same, except that great care should be taken to protect the neurovascular bundle when suturing the posterior horn. Accessing posterolateral exposure is necessary to safeguard the common peroneal nerve and to expose the lateral capsule. If there is any doubt about the suture placement, open posterior horn suturing should be performed in the standard fashion. Alternatively, meniscus and/or stabilization devices such as arrows or staples can be used instead of sutures. Stabilization arrows manufactured by Bionix, Inc., Malvern, Pa., are non-limiting examples of such stabilization arrows. Other stabilization devices known to those of ordinary skill in the art can also be used.

Routine skin closure and dressings are applied. Thirty milliliters of 0.5% Marcaine (Astra) with epinephrine may be instilled if desired. A postoperative hinged knee brace is applied with the range of motion limited to 30° of extension and 90° of flexion.

Postoperatively the patient is permitted partial weight bearing in a hinged knee brace for six weeks. The brace is removed for sleeping and out-of-brace range-of-motion exercises. On day one after surgery, exercises are initiated for quad strengthening, including leg raises, quad sets, and well-leg bicycling. After six weeks, knee-bend exercises, two-leg bicycling, and water running exercises are initiated. When maximal strength gains are achieved, pivoting sports can be resumed, usually at four to six months after surgery.

This invention is further illustrated by the following Examples which should not be construed as limiting. The contents of all references and published patents and patent applications cited throughout the application are hereby incorporated by reference.

EXAMPLE 1 Assay For α-Gal Epitopes' Elimination from Cartilage by α-Galactosidase

In this example, an ELISA assay for assessing the elimination of α-gal epitopes from cartilage is conducted.

A monoclonal anti-Gal antibody (designated M86) which is highly specific for α-gal epitopes on glycoproteins is produced by fusion of splenocytes from anti-Gal producing knock-out mice for α 1,3 galactosyltransferase, and a mouse hybridoma fusion partner.

The specificity of M86 for α-gal epitopes on glycoproteins is illustrated in FIG. 10. M86 binds to synthetic α-gal epitopes linked to -bovine serum albumin (BSA), to ▴-bovine thyroglobulin which has 11 α-gal epitopes, R. G. Spiro et al., Occurrence of α-D-galactosyl residues in the thyroglobulin from several species. Localization in the saccharide chains of complex carbohydrates, 259 J. Biol. Chem. 9858 (1984); or to ▪-mouse laminin which has 50 α-gal epitopes, R. G. Arumugham et al., Structure of the asparagine-linked sugar chains of laminin. 883 Biochem. Biophys. Acta 112 (1986); but not to □-human thyroglobulin or human laminin, ∘-Galβ-1-4 G1cNAc-BSA (N-acetyllactosamine-BSA) and Galα1-4Galβ1-4G1cNAc-BSA (P1 antigen linked to BSA), all of which completely lack α-gal epitopes. Binding is measured at different dilutions of the M86 tissue culture medium.

Once the M86 antibody is isolated, the monoclonal antibody is diluted from about 1:20 to about 1:160, and preferably diluted from about 1:50 to about 1:130. The antibody is incubated for a predetermined period of time ranging between about 5 hr to about 24 hr, at a predetermined temperature ranging from about 3° C. to about 8° C. The antibody is maintained in constant rotation with fragments of cartilage about 5 μm to about 100 μm in size, and more preferably with cartilage fragments ranging from about 10 μm to about 50 μm in size, at various cartilage concentrations ranging from about 200 mg/ml to about 1.5 mg/ml. Subsequently, the cartilage fragments are removed by centrifugation at centrifugation rate ranging from about 20,000×g to about 50,000×g. The proportion of M86 bound to the cartilage is assessed by measuring the remaining M86 activity in the supernatant, in ELISA with α-gal-BSA as described in the prior art in, for example, U. Galili et al., Porcine and bovine cartilage transplants in cynomolgus monkey: II. Changes in anti-Gal response during chronic rejection, 63 Transplantation 645-651 (1997). The extent of binding of M86 to the cartilage is defined as a percentage inhibition of subsequent binding to α-gal-BSA. There is a direct relationship between the amount of α-gal epitopes in the cartilage and the proportion of M86 complexed with the cartilage fragments, thus removed from the supernatant (i.e., percentage inhibition).

An example of the assay is shown in FIG. 11. Fragments of homogenized meniscus cartilage (∘) or meniscus cartilage (◯) treated with α-galactosidase are incubated with the M86 monoclonal antibody (diluted 1:100) for 20 hr at 4° C. Subsequently, the cartilage fragments are removed by centrifugation at 35,000×g and the remaining M86 in the supernatant is assessed in ELISA with α-gal-BSA as solid phase antigen. FIG. 11 shows that treatment of the cartilage with 200 U/ml α-galactosidase for 4 hour at 30° C. followed by five washes with phosphate-buffered solution (PBS) completely eliminates the α-gal epitopes. Thus, since there is no inhibition of subsequent M86 binding to α-gal-BSA even at a high cartilage fragment concentration of 200 mg/ml.

EXAMPLE 2 Assessment of Primate Response to Implanted Porcine Cartilage Treated with α-Galactosidase

In this example, porcine cartilage implants are treated with α-galactosidase to eliminate α-galactosyl epitopes, the implants are transplanted into cynomolgus monkeys, and the primate response to the cartilage implants is assessed.

Porcine stifle joints are sterilely prepared and the meniscus cartilage and surrounding attached soft tissues surgically removed. The cartilage specimens are washed for at least five minutes with an alcohol, such as ethanol or isopropanol, to remove synovial fluid and lipid soluble contaminants.

The cartilage specimens are frozen at a temperature ranging from about -35° C. to about -90° C., and preferably at a temperature up to about -70° C., to disrupt, that, is to kill, the specimens' fibrochondrocytes.

The cartilage is cut into two portions. The first portion is immersed in a buffer, such as citrate buffer solution, with a pH ranging from about 5 to about 6. The buffer contains α-galactosidase at a concentration ranging from about 50 U/ml to about 300 U/ml and an additive, such as PEG, ranging in a concentration of about 2% to about 6%. The cartilage/α-galactosidase buffer solution is incubated at a temperature ranging from about 25° C. to about 32° C. for a predetermined period of time ranging from about one hr to about six hr.

The second cartilage portion is incubated under similar conditions as the first cartilage portion in a buffer solution in the absence of α-galactosidase and serves as the control. At the end of incubation the cartilage is washed under conditions which allow the enzyme to diffuse out. For example, in the present example, the cartilage is washed twice with citrate buffer and three times with phosphate-buffered saline (PBS) pH 7.5. Each wash can include incubation in 50 ml of buffer solution for 10 min with gentle rocking at 24° C. Other washing procedures known to those of ordinary skill in the art can also be used.

Confirmation of complete removal of α-gal epitopes is performed using the ELISA inhibition assay with the monoclonal anti-Gal M86 antibody, as described above in Example 1. The α-galactosidase is produced according to the methods known in the prior art, such as, for example, the methods described in A. Zhu et al., Characterization of recombinant α-galactosidase for use in seroconversion from blood group B to O of human erythrocytes, 827 Arch. Biochem. Biophysics 324 (1996); A. Zhu et al., High-level expression and purification of coffee bean α-galactosidase produced in the yeast Pichia pastoris, 827 Arch. Biochem. Biophysics 324 (1996).

Each cartilage sample is implanted in the supra patellar pouch of six cynomolgus monkeys. With the animals under general inhalation anesthesia, an incision of about 1 cm is made directly into the supra patellar pouch at the superior medial border of the patella extending proximally. A piece of the porcine cartilage of about 0.5 cm to about 1 cm in length is placed into the pouch with a single 3-0 nylon stitch as a marking tag. The procedure is performed under sterile surgical technique, and the wounds are closed with 3-0 vicryl or a suitable equivalent known to those of ordinary skill in the art. The animals are permitted unrestricted cage activity and monitored for any sign of discomfort, swelling, infection, or rejection. Blood samples (e.g., 2 ml) are drawn periodically (e.g., every two weeks) for monitoring of antibodies.

The occurrence of an immune response against the xenograft is assessed by determining anti-Gal and non-anti-Gal anti-cartilage antibodies (i.e., antibodies binding to cartilage antigens other than the α-gal epitopes) in serum samples from the transplanted monkeys. At least two ml blood samples are drawn from the transplanted monkeys on the day of implant surgery and at periodic (e.g., two week) intervals post-transplantation. The blood samples are centrifuged and the serum samples are frozen and evaluated for the anti-Gal and other non-anti-Gal anti-cartilage antibody activity.

Anti-Gal activity is determined in the serum samples in ELISA with α-gal-BSA as solid phase antigen, according to methods known in the prior art, such as, for example, the methods described in Galili et al., Porcine and bovine cartilage transplants in cynomolgus monkey: II. Changes in anti-Gal response during chronic rejection, 63 Transplantation 645-651 (1997). For example, the α-gal-BSA antigen is used to coat ELISA microtiter wells. Subsequent to blocking of the wells with 1% BSA in PBS, sera is added to the wells in two fold serial dilutions, and incubated for 2 hr at room temperature. The plates are washed, and incubated with secondary anti-IgG antibody conjugated to peroxidase. Color reaction is performed with o-phenylenediamine. Anti-Gal activity at the various post-transplantation serum dilutions are compared with the baseline pretransplantation serum.

Assays are conducted to determine whether α-galactosidase treated xenografts induce the formation of anti-cartilage antibodies. For measuring anti-cartilage antibody activity, an ELISA assay is performed according to methods known in the prior art, such as, for example, the methods described in K. R. Stone et al., Porcine and bovine cartilage transplants in cynomolgus monkey: I. A model for chronic xenograft rejection, 63 Transplantation 640-645 (1997). For example, a solution of cartilage homogenate at 100 μg/ml in carbonate buffer is used as solid phase antigen. Other buffers known to those of ordinary skill in the art can also be used. Approximately 5 μg of cartilage antigens per well are dried and the wells are blocked with BSA. The serum samples used for this assay are depleted of anti-Gal by adsorption on rabbit red cells for 30 min at 4° C. (at 3:1 ration vol/vol). Under these conditions, all anti-Gal antibodies are adsorbed on the many α-gal epitopes expressed on rabbit red cells. U. Galili et al., Evolutionary relationship between the anti-Gal antibody and the Galα163Gal epitope in primates, 84 Proc. Natl. Acad. Sci. (USA) 1369 (1987); U. Galili et al., Contribution of anti-Gal to primate and human IgG binding to porcine endothelial cells, 60 Transplantation 210 (1995). The adsorbed sera at various dilutions are analyzed for anti-cartilage antibodies by ELISA, and the post-transplantation production of such antibodies are assessed by comparing this antibody activity with that observed in the pretransplantation serum.

The cartilage is optionally explanted at one to two months post-transplantation, sectioned and stained for histological evaluation of inflammatory infiltrates. Post-transplantation changes in anti-Gal and other anti-cartilage antibody activities are correlated with the inflammatory histologic characteristics (i.e., granulocytes or mononuclear cell infiltrates) within the explanted cartilage, one to two months post-transplantation, using methods known in the art, as, for example, the methods described in K. R. Stone et al., Porcine and bovine cartilage transplants in cynomolgus monkey: I. A model for chronic xenograft rejection, 63 Transplantation 640-645 (1997).

Where the cartilage is explanted, the cartilage xenograft is aseptically harvested, using anesthetic procedure, surgical exposure of the joint, removal of the implant and closure of the soft tissue. At the time of the xenograft removal, joint fluid, if present in amounts sufficient to aspirate, is collected from the stifle joints for possible immunologic testing if the gross and histopathologic evaluation of the transplants indicate good performance of the transplanted cartilage. Subsequently, the animals are allowed to recover and are monitored closely until the incisions have healed and the gait is normal. The xenograft samples are collected, processed, and examined microscopically. A portion of the implant and surrounding tissue is frozen in an embedding medium for frozen tissue specimens in embedding molds for immunohistochemistry evaluation according to the methods known in the prior art. "TISSUE-TEK®" O.C.T. compound which includes 10.24% w/w polyvinyl alcohol, 4.26% w/w polyethylene glycol, and 86.60% w/w nonreactive ingredients, and is manufactured by Sakura FinTek, Torrence, Calif., is a non-limiting example of a possible embedding medium for use with the present invention. Other embedding mediums known to those of ordinary skill in the art may also be used. The remaining implant and surrounding tissue is collected in 10% neutral buffered formalin for histopathologic examination.

EXAMPLE 3 Assessment of Primate Response to Implanted Cartilage Treated with α-Galactosidase, Sialic Acid-Cytosine Monophosphate and Sialyltransferase

In this example, porcine cartilage implants are treated with α-galactosidase to eliminate α-gal epitopes, as described in Example 2. The implants are further treated with sialic acid- cytosine monophosphate (SA-CMP) and sialyltransferase to cap carbohydrate chains with sialic acid. Sialytransferase facilitates the transfer of the sialic acid from the SA-CMP compound to the xenograft. The sialic acid links to and thus caps the carbohydrate chains. The cytosine monophosphate provides the necessary energetic level to the sialic acid for such linking and capping. Capping with sialic acid interferes with the ability of the subject's immune system to recognize the xenograft as foreign. The negative charge of the sialic acid further interferes with the ability of the cartilage antigens to bind with non-anti-Gal anti-cartilage antibodies (i.e., antibodies binding to cartilage antigens other than the α-gal epitopes.) The implants are transplanted into cynomolgus monkeys, and the primate response to the cartilage implants is assessed.

Porcine cartilage stifle joints are prepared as described in Example 2 including the α-galactosidase treatment. Prior to implantation into the monkeys, however, the implants are further treated with a predetermined amount of SA-CMP and sialyltransferase, at specified concentrations for a predetermined time and at a predetermined temperature, to cap carbohydrate chains with sialic acid. For example, the sample is immersed in 10 ml buffer solution at a pH of about 5.5 to 7.0, and preferably a pH of about 6.0-6.5, and most preferably a pH of about 6.2, containing SA-CMP at a concentration of approximately about 1 mM to about 10 mM, and sialyltransferase at a concentration of about 100 U/ml. The sample is incubated at a temperature range of about 26° C. to about 37° C. for a predetermined time period of about one hr to about four hr.

Other enzymes such as recombinant transialidase can be used to facilitate the transfer of sialic acid from compounds such as sialylated lactose to the xenograft.

Further, other molecules, such as fucosyl in combination with the corresponding fucosyltransferase and n-acetyl glucosamine in combination with the corresponding glycosyltransferase, can also be used for capping the carbohydrate chains of the implants.

Subsequently, the samples are washed to remove the enzyme and implanted into the monkeys, and the occurrence of an immune response against the xenograft is assessed as described above in Example 2.

Those of skill in the art will recognize that the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently described embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all variations of the invention which are encompassed within the meaning and range of equivalency of the claims are therefor intended to be embraced therein. 

What is claimed is:
 1. A method of preparing a meniscal xenograft for implantation into a human, which comprisesa. removing at least a portion of a medial or lateral meniscus from a knee joint of a non-human animal to provide a xenograft; b. washing the xenograft in water and alcohol; c. subjecting the xenograft to a cellular disruption treatment; d. digesting the xenograft with a glycosidase to remove substantially a plurality of first surface carbohydrate moieties from the xenograft, wherein the glycosidase has a concentration in a range of about 1 mU/ml to about 1000 U/ml; and e. treating a plurality of second surface carbohydrate moieties on the xenograft with a plurality of capping molecules to cap at least a portion of the second surface carbohydrate moieties, whereby the xenograft is substantially non-immunogenic and has substantially the same mechanical properties as the native meniscus.
 2. The method of claim 1, wherein the capping step comprises treating the second surface carbohydrate moieties on the xenograft with the capping molecules having a concentration in a range of about 0.1 mM to about 100 mM.
 3. The method of claim 1, wherein at least a portion of the capping molecules are sialic acid molecules.
 4. The method of claim 1, wherein the glycosidase is a galactosidase.
 5. The method of claim 4, wherein the galactosidase is an α-galactosidase.
 6. The method of claim 1, wherein the cellular diruption treatment comprises freeze/thaw cycling.
 7. The method of claim 1, wherein the cellular disruption treatment comprises exposure to gamma radiation.
 8. The method of claim 1, wherein the removing step comprises removing the portion of the medial or lateral meniscus having a superior principal surface and an inferior principal surface, each of the principal surfaces having an outer portion being joined by an outer lateral surface, and each of the principal surfaces having an inner portion being joined by an inner lateral surface.
 9. The method of claim 1 further comprising the step of following step c, exposing the xenograft to a crosslinking agent in a vapor form.
 10. A method of preparing a meniscal xenograft for implantation into a human, which comprisesa. removing at least a portion of a medial or lateral meniscus from a knee joint of a non-human animal to provide a xenograft; b. washing the xenograft in water and alcohol; c. subjecting the xenograft to a cellular disruption treatment; d. digesting the xenograft with a glycosidase to remove substantially a plurality of first surface carbohydrate moieties from the xenograft; and e. treating a plurality of second surface carbohydrate moieties on the xenograft with a plurality of sialic acid molecules to cap at least a portion of the second surface carbohydrate moieties,whereby the xenograft is substantially non-immunogenic and has substantially the same mechanical properties as the native meniscus.
 11. The method of claim 10, wherein the capping step comprisestreating the second surface carbohydrate moieties on the xenograft with the sialic acid molecules having a concentration in a range of about 0.01 mM to about 100 mM.
 12. The method of claim 10, wherein at least the glycosidase is a galactosidase.
 13. The method of claim 12, wherein at least the galactosidase is an α-galactosidase.
 14. The method of claim 10, wherein the cellular disruption treatment comprises freeze/thaw cycling.
 15. The method of claim 10, wherein the cellular disruption treatment comprises exposure to gamma radiation.
 16. The method of claim 10, wherein the removing step comprises removing the portion of the medial or lateral meniscus having a superior principal surface and an inferior principal surface, each of the principal surfaces having an outer portion being joined by an outer lateral surface, and each of the principal surfaces having an inner portion being joined by an inner lateral surface.
 17. The method of claim 10 further comprising the step offollowing step c, exposing the xenograft to a crosslinking agent in a vapor form.
 18. An article of manufacture comprising a substantially non-immunogenic knee meniscal xenograft for implantation in to a human, produced bya. removing at least a portion of a medial or lateral meniscus from a knee joint of a non-human animal to provide a xenograft; b. washing the xenograft in water and alcohol; c. subjecting the xenograft to a cellular disruption treatment; d. digesting the xenograft with a glycosidase to remove substantially a plurality of first surface carbohydrate moieties from the xenograft, wherein the glycosidase has a concentration in a range of about 1 mU/ml to about 1000 U/ml, and e. treating a plurality of second surface carbohydrate moieties on the xenograft with a plurality of capping molecules to cap at least a portion of the second surface carbodydrate moieties on the xenograft,whereby the xenograft is substantially non-immunogenic and has substantially the same mechanical properties as the native meniscus.
 19. The article of manufacture of claim 18, wherein the capping molecules have a concentration in a range of about 0.1 mM to about 100 mM.
 20. The article of manufacture of claim 18, wherein at least a portion of the capping molecules are sialic acid molecules.
 21. The article of manufacture of claim 18, wherein the glycosidase is a galactosidase.
 22. The article of manufacture of claim 21, wherein the galactosidase is an α-galactosidase.
 23. The article of manufacture of claim 18, wherein the cellular disruption treatment comprises freeze/thaw cycling.
 24. The article of manufacture of claim 18, wherein the cellular disruption treatment comprises exposure to gamma radiation.
 25. The article of manufacture of claim 18, wherein the removing step comprises removing the portion of the medial or lateral meniscus having a superior principal surface and an inferior principal surface, each of the principal surfaces having an outer portion being joined by an outer lateral surface, and each of the principal surfaces having an inner portion being joined by an inner lateral surface.
 26. The article of manufacture of claim 18 further comprising the step offollowing step c, exposing the xenograft to a crosslinking agent in a vapor form.
 27. An article of manufacture comprising a substantially non-immunogenic knee meniscal xenograft for implantation in to a human, produced bya. removing at least a portion of a medial or lateral meniscus from a knee joint of a non-human animal to provide a xenograft; b. washing the xenograft in water and alcohol; c. subjecting the xenograft to a cellular disruption treatment; d. digesting the xenograft with a glycosidase to remove substantially a plurality of first surface carbohydrate moieties from the xenograft; and e. treating a plurality of second surface carbohydrate moieties on the xenograft with a plurality of sialic acid molecules to cap at least a portion of the second surface carbohydrate moieties,whereby the xenograft is substantially non-immunogenic and has substantially the same mechanical properties as the native meniscus.
 28. The article of manufacture of claim 27, wherein the sialic acid molecules have a concentration in a range of about 0.01 mM to about 100 mM.
 29. The article of manufacture of claim 27, wherein the glycosidase is a galactosidase.
 30. The article of manufacture of claim 29, wherein the galactosidase is an α-galactosidase.
 31. The article of manufacture of claim 27, wherein the cellular disruption treatment comprises freeze/thaw cycling.
 32. The article of manufacture of claim 27, wherein the cellular disruption treatment comprises exposure to gamma radiation.
 33. The article of manufacture of claim 27, wherein the removing step comprises removing the portion of the medial or lateral meniscus having a superior principal surface and an inferior principal surface, each of the principal surfaces having an outer portion being joined by an outer lateral surface, and each of the principal surfaces having an inner portion being joined by an inner lateral surface.
 34. The article of manufacture of claim 27 further comprising the step offollowing step c, exposing the xenograft to a crosslinking agent in a vapor form.
 35. A meniscal xenograft for implantation into a human comprisinga portion of a medial or lateral meniscus from a knee joint of a non-human animal, wherein the portion includes an extracellular matrix and a plurality of substantially only dead cells, the extracellular matrix and the dead cells having substantially no surface α-galactosyl moieties and having a plurality of sialic acid molecules linked to at least a portion of a plurality of surface carbohydrate moieties on the xenograft, whereby the portion of the medial or lateral meniscus is substantially non-immunogenic and has substantially the same mechanical properties as the native meniscus.
 36. The meniscal xenograft of claim 35, wherein the portion comprises a superior principal surface and an inferior principal surface, each of the principal surfaces having an outer portion being joined by an outer lateral surface, and each of the principal surfaces having an inner portion being joined by an inner lateral surface. 